1. Field of the Invention
The present invention relates generally to wellbore cleanout techniques and, more particularly, to cleaning out horizontal wellbores using coiled tubing.
2. Description of the Related Art
In typical annular coiled tubing fracturing techniques, operators run a bottom hole assembly (“BHA”) in the well with coiled tubing, perforate the casing, pull the bottom hole assembly past the build section into the vertical section of the well or even out of the well entirely, and then begin pumping fracturing fluid. A sand plug is then set as the final stage of the fracture pumping operation, the BHA is run back in hole to a desired location above the previous interval and another interval is perforated. The BHA is then pulled back into the vertical section or out of the well, and fracturing commences again. Accordingly, there are multiple trips in and out of the well. In some instances, the use of mechanical bridge plugs is preferred over sand plugs; when this is the case, the BHA must be fully removed from the well after each perforating operation, in order to pump the fracturing fluid and, at the same time, fit a new plug on the bottom of the perforating BHA.
It is economically desirable to perforate, fracture and isolate each interval quickly, so that all intervals can be treated in the shortest time possible, preferably within a day. However, the nature of horizontal wells creates beds of solids, such as sand or proppant, which settle on the bottom side of the horizontal section, introducing additional complexities into the reliable execution of the fracturing process. In a horizontal well, gravity causes the coiled tubing to sit on the lower side of the well, creating an eccentric flow channel along the annulus. As fluid flows down the annulus, a region of high shear is created around the coiled tubing, which causes proppants in the fracturing fluid to settle on the low side of the hole, creating a proppant bed, for example. Given that a horizontal section could spans thousands of feet, several thousand pounds of proppant can be deposited along the bottom of the horizontal section.
The proppant bed must be removed before the next perforated interval is fractured. If the proppant bed is not cleaned, the pad for the next fracture can entrain this proppant, thereby creating a high potential for premature screen out.
Generally, there are three methods of removing solids from wells. The first method, called “stationary circulating,” involves circulating clean fluid down the coiled tubing and blowing the proppant up the annulus until all the proppant has been transported out of the well while the coiled tubing is stationary. The second method, called a “wiper trip,” involves circulating down the coil and washing the proppant back up the annulus while pulling the BHA out of the hole. The third method, called reversing, involves circulating down the annulus and washing proppant up the coil while the BHA is running in hole.
There are disadvantages to the traditional cleaning methods. In a typical 10,000 ft horizontal well, using 2″ coiled tubing it would take about 6 hours to remove the proppant using the circulating method. In the same well using the wiper trip, the clean out would take about 3 hours. Moreover, since wiper tripping requires pulling the BHA practically all the way to surface, operators then have to run back into the hole in order to perforate the next interval and continue fracturing. Lastly, using the reversing method, a clean out could take somewhere in the range of 1½ hours which, while faster than the other two methods, is still time consuming and negatively impacts overall process efficiency.
In view of these disadvantages, there is a need in the art for an improved method for cleaning out a horizontal well which substantially reduces the cleanout time, allowing more intervals to be fractured in a day.